Spinal fixation devices are used to stabilize vertebrae in need of stabilization. For example, surgically fused vertebrae can benefit from such stabilization. Conventional devices, known for use in the lumbar region of the spine, generally employ spinal support assemblies. Such spinal support assemblies typically include a reinforcing rod, and an attachment adapter assembly. A typical attachment adapter assembly includes an adapter which extends generally between the reinforcing rod and a vertebra which provides a supporting foundation from which the vertebra or vertebrae being treated can be supported. A typical such spinal support assembly commonly employs two or more adapter assemblies, which support the spinal support assembly from two or more such foundation support vertebrae.
The adapter can include a lamellar hook which can be used to secure the adapter to a such support vertebra. In the alternative, the adapter assembly can include one or more e.g. titanium bone screws as elements separate and distinct from the adapter, itself, or loosely mounted to the adapter. The bone screws are used to secure the adapter to the support vertebra or vertebrae.
A typical reinforcing rod is a titanium rod having a nominal diameter of about 6 mm, and length sufficient to extend along the vertebrae being fused, and at least to the next adjacent vertebrae which can be used as the foundation support vertebrae. Such length provides sufficient purchase on the rod, by the adapter assemblies, to enable the surgeon to securely attach the adapters to the reinforcing rod, as well as to the support vertebrae. The rod diameter can, of course, be specified greater or less than the above-noted 6 mm as an adaptation to the general size and strength needs associated with the particular subject being treated. Such size and strength needs can vary according to the size of the subject or spine, or can vary according to the species being treated in the case of treatment of e.g. non-human subjects.
Attachment of the adapter/rod combination to adjacent vertebrae, e.g. foundation support vertebrae, by means of the adapters, and which support vertebrae are adjacent the vertebrae being treated and/or supported, and which provide the basis for supporting the rod, is known. Especially useful attachment combinations, namely reinforcing rod, adapter, and securing device, e.g. bone screw or hook, are taught in my U.S. Pat. No. 6,478,797, the disclosure of which is hereby incorporated herein by reference in its entirety.
The purpose of a spinal support assembly is to provide support to vertebrae which are being treated, typically vertebrae which are being fused to each other. However, in providing such support to the vertebrae which are being fused, such support is inherently a transfer of forces to and from the adjacent foundation, support vertebrae, by the spinal support assemblies, whereby at least some of the forces, which would normally be imposed on the vertebrae being treated, by-pass those treated vertebrae through the spinal support assembly.
For example, the forces which by-pass the vertebrae being treated can be characterizable as both vertical support loads and angularly-directed lateral loads.
Restated, the purpose of the spinal support assembly is to provide control of forces which would otherwise be applied to the vertebrae being treated. Further, a purpose is to serve as a force by-pass route once the spinal support assembly has been surgically attached to the support vertebrae. Movement of that portion of the spine to which the spinal support assembly is attached is effectively controlled by the inherent stiffness and rigidity of the reinforcing rod in combination with the stiffness and rigidity of the associated attachments, and any capacity for movement at the respective interfaces internal to the spinal support assembly, and between the spinal support assembly and the foundation support vertebrae. In known spinal support assemblies, the treated portion of the spine is effectively immobilized by the above-exemplified stiffnesses and rigidities.
The objective sought to be achieved by the surgical procedure wherein the forces are re-routed through the spinal support assembly, rather than accepting passage of such forces through the vertebrae being treated, is to neutralize or attenuate pain and dysfunction which is otherwise associated with the vertebrae which are to be fused. Such pain can be associated with neurocompression and/or with pain associated with loading and/or moving pain-generating tissues such as associated discs, facet joints, ligaments, muscles, or fracture fragments.
A problem associated with spinal support assemblies known in the art is that pain relief achieved by installing the spinal support assembly is achieved at the cost of effectively immobilizing that portion of the spine; namely immobilizing that portion of the spine both vertically and laterally. Such immobilizing of the spine correspondingly limits mobility of the patient. In addition, such immobilizing of the spine limits the ability of the associated foundation support vertebrae to adjust to the imposition of the forces so transferred, whereby the foundation support vertebrae can, over time, suffer accelerated degradation of their own capabilities.
The applicant herein contemplates that, in some instances, the need for stabilization of the respective vertebrae being treated does not necessarily correspond with a need for the affected components of the spine to be completely immobilized. Rather, in such instances, maintaining at least lateral mobility of the portion of the spine which is being treated can provide benefit to the subject being treated, greater than if the spine were conventionally immobilized against all movement. Namely, the subject retains at least some mobility of the fused portion of the spine.
In addition to improved mobility of the subject being treated, mobility of the affected portion of the spine enables the foundation support vertebrae to better adjust, in real time, to the dynamics of the forces being imposed on that portion of the spine. Such real-time adjustment to such forces, by the foundation support vertebrae, enhances prospects for the foundation support vertebrae to provide normal levels of vertebral support to the subject for normal life-cycle periods of time.
There is therefore a need for spinal stabilization apparatus which accommodates limited and controlled motion of the treated portion of the spine, while providing the required level of stabilization of the treated portion of the spine to support the dynamics of body forces being imposed on that portion of the spine by the subject's/patient's body, all in the context of controlling, limiting, the spatial volume of material which is being attached to the spine.
There is a corresponding need for spinal stabilization apparatus which accommodates limited lateral or angular movement of the treated portion of the spine while stabilizing the treated portion of the spine against transverse lateral movement and limiting the extent of vertical movement of the treated portion of the spine.
There is a corresponding need for spinal stabilization apparatus which accommodates limited and controlled motion of the treated portion of the spine while providing the required level of stabilization of the treated portion of the spine, all in the context of minimizing the amount of space which is occupied by such spinal support apparatus.
Specifically, there is a need for spinal stabilization apparatus which accommodates limited lateral angular movement of the treated portion of the spine while stabilizing the treated portion of the spine against transverse lateral movement, and limiting the extent of vertical movement of the treated portion of the spine.
There is a yet further need to provide such spinal stabilization apparatus in a format and structure which follows closely the known configuration of a single reinforcing rod supported by first and second adapter assemblies, wherein the adapter assemblies provide the interface between the reinforcing rod and the foundation support structure.